In processes for molding thin-walled plastic shells, a thermoplastic resin material is fed through an opening in a powder box to a mold surface of a mold box. Generally, the mold box and powder box are operatively connected such that the mold surface of the mold box is brought into close proximity of the opening in the powder box. The resin material is fed over the mold surface of the heated mold box and is melted into a molten resin film. The film is cooled to obtain the thin-walled plastic shell and is removed from the mold surface.
Currently, this molding process is accomplished with the powder box and mold box sealing where the two units meet; that is, at the top of the liner about the opening of the powder box and near the termination of the run off of the mold surface. This process allows the powder resin to contact the surface areas of the mold where such contact is not desired, causing waste and necessitating trimming of the finally formed part. Other methods known in the art of blocking this exposure, such as by using various insulating materials on the mold surface, still pose the problem of having to adopt special impacting and/or cooling, and/or brushing methods to keep them clean.
For example, the U.S. Pat. No. 4,790,510 to Takamtasu et al, issued Dec. 13, 1988, discloses a slush mold apparatus and method for molding film moldings such as surface coverings of interior parts of automotive vehicles. The powder box includes a skirt portion connected thereto having a lower edge which contacts the mold surface about its upper outer periphery. The skirt is a solid member which must be uniquely adapted to each unique mold surface configuration.
U.S. Pat. No. 4,773,844 discloses two mold boxes, each having a skin of thermoplastic material formed thereon. The mold boxes are then joined at margins under light pressure so as to separate salvage from the joined skins.
Presently there are two other methods being used to "control" thermoplastic resin powder flow (deposition on the shell mold surface) during the molding process. Only the second method selectively controls the powder placement.
One method grossly controls the flow of such powder, e.g., polyvinylchloride. The powder box is simply sealed to the shell mold on a perimeter flange. All mold surfaces interior to the powder box gasket are allowed to contact the powder thus forming (molding) a shell on the entire mold surface. The drawback to this method is that, on the average, 50% of the mold surface is not actual finished part surface (sometimes referred to as Class "A" surface). The shell molded on non-Class "A" areas is scrap material.
Another control method uses insulators mounted on non-Class "A" mold surfaces. The insulators and modulation of heat in these areas reduce the temperature of the exposed surface to the powder to a degree where the powder does not adhere, thus shell is not formed in the areas of the insulators. Such insulators are shown in U.S. Pat. Nos. 4,683,098 and 4,716,003 owned by the assignee of the present invention.
The present invention provides an apparatus and method for sealing the mold about its periphery along a variable height narrow width band to form a narrow flange on the thin-wall shell molded therein. Accordingly, the present apparatus and method generate little or no waste and trimming is reduced. Further, the present invention provides an apparatus and method for providing such a narrow width band portion seal between the opening of the powder box and the periphery of the mold surface at deep recesses within the mold. Such deep mold surfaces are formed in a mold box such that common mold box peripheries cannot extend into the mold surface to contact the periphery of the mold surface about the shell to be formed and further have surfaces thereon in which waste thermoplastic powder can build up when the mold is uniformly heated. The present invention provides an apparatus and method for perfecting such a seal in deep molds as well as in more shallow molds by use of a core device connected to the open edge of a powder box and having a variable height edge that conforms to either deep or shallow molds. Such a device can be adapted for both deep and shallow molds such that a single device is not limited to either deep or shallow mold use and the device is adapted to carry a gasket retainer and seal gasket with a perimeter shaped to form a variable height small width band seal to form a narrow flange at variable heights on the molded shell.